In an LCD, various voltage signals are applied to LCD elements to change states of liquid crystal so as to change transmittance and gray or color levels. Take a 256-level display as an example, the 256 levels are indicated by 8 bits, and as shown in the plot of FIG. 1, voltage values in the vertical axis respectively corresponding to gray/color levels 0˜255 in the horizontal axis are selectively applied to the LCD pixels.
Generally, data are updated every frame in an LCD. Viewing from a single LCD pixel, transmittance readily varies with a given level data and an applied voltage. However, the response speed of liquid crystal is not definitely quick as well. Response speed is typically defined by a period of time required for achieving 10%˜90% of expected luminance.
Generally, response speed significantly decreases in a low-temperature environment. A machine like a vehicular navigation system used in for example Northern Europe even possibly needs to be started in a temperature as low as minus tens of Centigrade degrees. In such a low temperature, liquid crystal is too viscous to be well responsive while starting. Therefore, the resulting image is vague and poor displaying quality is rendered.
A method having been developed for enhancing response speed of liquid crystal is known as “overdrive”. An overdrive method is a technique applying a voltage higher than a voltage determined according to a given data level, e.g. 0˜255, to accelerate the change of the LC state. The higher voltage, for example, is a voltage corresponding to a level higher than the given data level.
FIG. 2A and FIG. 2B schematically illustrate conventional overdrive methods. In the plots, the horizontal axis represents frame numbers, wherein each frame period is about 16.7 ms when driven under 60 Hz, and the vertical axis represents voltages respectively corresponding to gray/color levels. In an 256-level example, the level corresponding to black is defined as 0 and the level corresponding to white is defined as 255.
Referring to FIG. 2A, by general driving, the level of target 1 cannot be achieved until 10 frames or more pass. On the other hand, by overdriving with a voltage corresponding to a higher level OD1, the level of the target level 1 is achieved after 5 frames, as shown in the curve OD1. It is apparent that the response feature is improved. Therefore, it is feasible to reduce the time taken for achieving the target level 1 by applying an overdrive voltage OD1 during the first 5 frames and then applying a general drive voltage OD1′ corresponding to the target level 1, as shown in FIG. 2B.
Likewise, as shown in FIG. 2A, if the target level 2 is to be achieved after 5 frames, a voltage corresponding to a higher level OD2 than the target level 2 is applied. In other words, a steeper curve OD2 is adopted in order to achieve the target level 2 after the same 5 frames. Afterwards, a general drive voltage OD2′ corresponding to the target level 2 is applied, as shown in FIG. 2B. In this manner, the time taken for achieving the target level 2 can be reduced.
It is understood from the above that by way of overdriving, the states of liquid crystal molecules can be changed more quickly than by general driving so as to improve response property.
The overdrive method can be applied to achieve any desired target level, including the white level involving the steepest overdrive curve, after a predetermined number of frames, e.g. an arbitrary number more than 1.
Unfortunately, the conventional overdrive method is inefficient at a freezing low-temperature. For example, at −30° C., it requires about 100 frame periods to change from black to white. The overdrive operation for the first 5 frames shows almost no effect.
On the other hand, at a normal temperature, the same overdrive operation repetitively performed for a specified number of frames would deteriorate the following feature to previous images.
For precisely controlling overdrive voltages depending on images, another conventional overdrive method is proposed to predict level data for each pixel in the previous frame and then output overdriven level data accordingly, as disclosed in Japanese Patent Publication No. 2005-107531.
Since the overdrive operation in Japanese Patent Publication No. 2005-107531 is updated every frame, and it is known the level change between adjacent frames could be insignificant, the predicted values are likely to have no or almost no change. Then the overdrive effect cannot be seen.